A basement membraneassociated protein of restricted tissue specificity has been detected using monoclonal antibodies and named merosin. Merosin is present only in association with basement membranes (BM) of placenta (trophoblast BM), muscle (muscle fiber BM), and peripheral nerve (Schwann cell BM), as evidenced by indirect immunofluorescence. A fragment of merosin of about 55 kd was isolated from a pepsin digest of placenta and used to raise a polyclonal rabbit antiserum to merosin. The rabbit antibodies stain human tissues identically to the monoclonal antibodies and also react with muscle and peripheral nerve BMs in mouse tissues. The intact, tissue form of merosin appears to have a size of about 75 kd and may interact directly or indirectly with laminin. We will obtain a complete amino acid sequence of merosin by a combination of protein and cDNA sequencing. The 55 kd pepsin fragment of merosin, which can be isolated in milligram amounts, will be used to obtain protein sequence information. cDNA clones will be selected from an existing placenta lambdagtll cDNA library using specific antibodies and synthetic oligonucleotides. Intact merosin will be isolated from placental extracts and used to study the interaction of merosin with other BM and extracellular matrix proteins and with cells. The biosynthesis of merosin will be studied in cell and tissue culture to obtain information on size and number of subunits of the mature protein and if it is processed before being deposited in the extracellular matrix. The localization of merosin in relationship to the basement membrane in normal adult tissue will be studied using immunoelectron microscopy. Developmental expression of merosin will be studied in mouse by immunofluorescence. Similarly, the presence of merosin in placentas form different species and in tumors in man will be investigated. The expression of merosin will be correlated with the particular functions and with the degree of differentiation of the cells and tissues expressing merosin. This study may yield important information on tissuespecific functions of BMs.